Enemy series (Charlie Higson)

A terrible disease has struck everyone sixteen and over, leaving them either dead or a decomposing, flesh-eating creature. The action starts in a boarding school just outside London, where all the teachers have turned into sickos. A few kids survive and travel by bus into the city. The bus driver, an adult named Greg, seems to be unaffected by the disease. Then he begins to show the dreaded signs: outer blisters and inner madness. The kids escape...

The sickness destroyed everyone over the age of fourteen. All across London diseased adults are waiting, hungry predators with rotten flesh and ravaged minds. Small Sam and The Kid, have survived, safe with Ed at the Tower of London, but Sam is desperate to find his sister. Their search for Ella means Sam and The Kid must cross the forbidden zone into something more terrifying than any of the horror they've suffered so far.

Ella is out in the country, alone now except for her silent rescuer, Scarface, about whom she knows nothing, while Ed leaves London on a dangerous quest, determined to find Ella and keep his promise to Small Sam that he will reunite sister and brother.